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Internal GPS: Sea turtles navigate vast ocean to find the beach where they were born, but how?

It’s a sea turtle mystery that has stumped scientists for decades: How does the female sea turtle, which travels across thousands of miles of open ocean each year, still manage to navigate back to the same beach where she hatched to lay her eggs?

Keep in mind that there are no visual guideposts in the open ocean where sea turtles spend most of their lives. It is vast and featureless – an expanse of blue. And yet every two to three years, sea turtles dig their nests at the same location where they once crawled out of their own leathery eggs – a behavior known as natal homing.

Scientists have hypothesized that sea turtles may rely on information in the Earth’s geomagnetic field to help them find their way back to their birth beach, but they had never been able to find any evidence to support this hypothesis. Until now.

In a paper published last month in Current Biology, researchers show that subtle changes in the Earth’s magnetic field affect where loggerhead sea turtles bury their eggs on the Florida coast, providing the first clue that what’s known as the geomagnetic imprint hypothesis may be correct.

Since the mid-1990s, scientists have known that sea turtles are capable of deriving navigational information from the Earth’s geomagnetic field – a field humans cannot sense without the use of scientific instruments. The magnetic field’s intensity is strongest at the poles, and weakest at the equator. It also intersects the Earth at different angles, which are known as inclination angles.

“Turtles have evolved a way to use the inclination angle of the geomagnetic field, and its intensity, to give them almost an internal GPS,” said J. Roger Brothers, a graduate student at the University of North Carolina and the first author on the paper.

To see whether female turtles rely on this navigational tool to find their home beaches, Brothers and his adviser, UNC marine biologist Kenneth Lohmann, examined data collected over 19 years by the Florida Fish and Wildlife Conservation Commission on loggerhead nesting sites on the state’s Atlantic Coast, the largest sea turtle rookery in North America.

Because the Earth’s magnetic field is in constant flux, the researchers hypothesized that in years when the magnetic signatures of adjacent beaches moved closer together, there should be more nests on the same stretch of beach. In years when the magnetic signatures move farther apart, the nests should be more spread out.

That’s exactly what they found. At times and places where the magnetic signatures converged, scientists found an average increase of 35 percent in nesting density. At times when the signatures diverged, nesting density decreased by an average of 6 percent.

Nathan Putman, a research associate with the National Research Council who studies marine animal movement, said a turtle’s instinct to return to her home beach to nest is an evolutionary solution to a fundamental problem: How can you be sure your child will hatch in a safe place, find good foraging grounds, benefit from favorable current systems and avoid being eaten by predators?

“What they hit on is you go back to the beach where you were hatched,” he said. “If it was good enough for you, it is good enough for your offspring.”

Putman said the study provides the strongest evidence to date for any of the hypotheses about how sea turtles return to their natal site.

“It is just a correlation but the statistics they have are robust,” he said. The probability that the changes in Earth’s magnetic signatures and the turtles’ nesting behavior are merely coincidental is “quite low.”

Brothers and Lohmann do not suggest that the turtles are relying solely on a geomagnetic map to lay their eggs. There are other questions to be answered as well.

For example, scientists are still not sure what mechanism the turtles are using to detect the geomagnetic field. “Most likely they have tiny magnetic particles in their brains or in their bodies that act like a compass, but conclusive evidence is lacking,” Brothers said.